The present invention relates generally to the field of semiconductor structures, and more particularly to the inducement of strain on field-effect transistor channels.
A pure semiconductor is a poor electrical conductor as a consequence of having just the right number of electrons to completely fill its valence bonds. Through various techniques (e.g. doping or gating), the semiconductor can be modified to have an excess of electrons (becoming an n-type semiconductor) or a deficiency of electrons (becoming a p-type semiconductor). In both cases, the semiconductor becomes much more conductive (the conductivity can be increased by one million-fold or more). Semiconductor devices exploit this effect to shape electrical current. The study of semiconductor materials is an important area of material science research due to their application in devices such as transistors and therefore computers.
The most commonly used semiconductor materials are crystalline inorganic materials, which are classified according to the periodic table groups of their constituent atoms and also whether they are composed of a single element or more than one element. For example, silicon (Si) is a common semiconductor material and it is a group IVA element so it is classified as a group IV elemental semiconductor. Silicon-germanium (SiGe) is an alloy of two different group IVA elements so it is classified as a group IV compound semiconductor. When a semiconductor is composed of two or more elements from different periodic table groups, indicating compound vs. element is no longer necessary. Thus, lead sulfide (PbS) is composed of a group IVA element (Pb) and a group VIA element (S) so it is referred to as a IV-VI semiconductor. Likewise, indium phosphide (InP) is composed of a group IIIA element (In) and a group VA element (P) so it is a III-V semiconductor. There are several other classes of semiconductors of varying popularity (e.g., II-VI, I-VII, V-VI, II-V). Finally, semiconductors composed of two different elements are binary (e.g., InP), semiconductors with three different elements are ternary (e.g. the III-V semiconductor indium gallium arsenide (InGaAs)), and those with four and five different elements are quaternary and quinary, respectively.
Doping is the introduction of impurities to a semiconductor in order to vary its electronic nature. Depending on how it is doped, a semiconductor can be made to be n-type or p-type. For example, Si can be made n-type by doping with phosphorus. Phosphorus (P) has one more valence electron than Si and its incorporation into the Si crystal lattice creates a preponderance of nonbonding electrons, which are available as negative charge carriers. The “n” in n-type stands for “negative” and indicates that electrons are the majority charge carriers. If Boron were used as the dopant instead of P, the Si would become a p-type semiconductor. Boron (B) has one less valence electron than Si, which means it can only form covalent bonds with three Si atoms in the Si crystal lattice. The absence of an electron where one could exist in an atomic lattice is referred to as a hole. Electrons can migrate hole-to-hole leaving a hole behind each time. Thus, p-type semiconductors have moving holes as majority charge carriers and the “p” stands for “positive.”
Field-effect transistors (FETs) are transistors that employ an electric field to control the conductivity of a channel in which one of the two types of charge carriers may travel. A FET is composed of a source and a drain connected by the channel through which the charge carriers, electrons or holes, pass when voltage is applied to a gate. The gate sits over the channel separated by an insulating material referred to as the gate dielectric. Applying voltage to the gate changes the amount of charge carriers in the channel thereby controlling the current in the device.
Charge carrier transport that is typically described by mobility through FET channels is an important factor for optimal performance. One way charge carrier transport can be modulated is through strain. For example, strained channels have been successfully integrated into Si- and Ge-based metal oxide semiconductor FETs (MOSFETs) to enhance carrier mobility. Strain can be in the form of biaxial or uniaxial strain and may be compressive or tensile. A biaxial strained crystal has stress introduced in two directions (x-y) along its surface whereas a uniaxial strained crystal has stress introduced in only one direction. Compressive strain occurs when the crystal lattice is being compressed whereas tensile strain occurs when the crystal lattice is being stretched. In silicon, compressive strain facilitates hole mobility and tensile strain facilitates electron mobility.
Enhancing charge carrier transport for FETs through the use of uniaxial strain is an ongoing challenge. For example, high mobility, narrow band gap III-V materials are considered strong contenders to replace strained-Si channels. However, even though these III-V materials demonstrate high electron mobility, they are less promising for hole transport due to intrinsic lower hole mobility when compared with strained-Si. Eventual integration of III-V based semiconductors as MOSFET channels requires both high performance n- and p-MOSFETS. Thus, methods for fabricating III-V-based p-FETs with uniaxial compressive strained channel regions in order to improve hole transport is of significant interest.